A control arm (also called a “suspension arm”) is an arm for controlling the movement of wheels. The control arm is an important component that cushions the impact from the road surface and determines not only the riding comfort but also the travel performance and vehicle height and the like.
The manner in which an I-type control arm 1 is mounted to the suspension of an automobile will now be described using FIG. 15. FIG. 15 illustrates the control arm 1 viewed down from an upper part of the vehicle body.
As illustrated in FIG. 15, the control arm 1 is a component that has a long shape and connects a vehicle body 2 and an axle 3. More specifically, the control arm 1 is a component for positioning the axle 3 in the longitudinal direction or the transverse direction of the body 2, and for also enabling movement of the axle 3 with respect to the body 2 in the vertical direction.
The control arm 1 is amiably attached to the body 2 and the axle 3 through connection brackets 4 or connection tubes (not illustrated), for example. When the axle 3 displaces in the vertical direction, the control arm 1 controls the motion of tires 5 in a suitable and supple manner by turning with respect to the body 2 and the axle 3 and also by torsional deformation of the main body portion. By this means, the grip of the tires 5 can be improved, and the steering stability of the automobile when turning is enhanced. Further, an impact that the body 2 receives when the tires 5 run over a step such as a sidewalk can be absorbed.
The currently available types of control arms include the I-type arm (hereunder, referred to as “I-arm”) 1 that has an I-shaped external shape as illustrated in FIG. 15, and an A-type arm that has an A-shaped external shape. Of these two types, the I-arm 1 is adopted in many kinds of vehicles, especially in the kinds of vehicle that have a link-type suspension, because the structure of the I-arm is simple and consequently an increase in production costs can be suppressed. Although a round steel bar or a steel pipe has conventionally been used for the main body portion of the I-arm 1, the round steel bar has a large weight, while if a steel pipe is used the cost increases.
As described above, the I-arm 1 is an extremely important component for improving the steering stability and riding comfort. In order to reliably position the axle 3 in the longitudinal direction or transverse direction of the body 2, there is a need to provide the I-arm 1 with a predetermined tensile strength and compressive strength. In addition, in order to cause the axle 3 to displace in a supple manner in the vertical direction, the I-arm 1 is required to have a characteristic such that, when torsional stress is applied to the I-arm 1 as the axle 3 displaces in the vertical direction, the I-arm 1 flexibly and favorably causes torsional deformation to occur without generating bending deformation. Therefore, a large number of inventions with elaborated cross-sectional shapes of the I-arm 1 have already been proposed.
For example, an I-arm with a circular cross-section (Patent Document 1), an I-arm with a cross-sectional W-shape (Patent Document 2), an I-arm with a cross-sectional U-shape (Patent Documents 3 and 4, see FIG. 14(b)) and the like have been proposed. In addition, with respect to the lower arm of an automobile in which a stabilizer mounting portion is provided between a wheel-side mounting portion of the lower-arm main body and a body-side mounting portion, a configuration has been proposed in which an approximately cross shape is provided on a mounting portion side of the body side in the lower arm of an automobile (see Patent Document 5, FIG. 13(b)).